1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and more particularly, to a channel estimation method and system for estimating a channel in the time domain by calculating a linear correlation and repeatedly canceling adjacent channel interference using Decision-Feedback-Equalization (DFE).
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation, in which data to be transmitted is converted into an M-ary QAM (Quadrature Amplitude Modulation) modulated complex symbol or a sequence of complex symbols. A complex symbol sequence is converted into a plurality of parallel (simultaneous) complex symbols through serial-to-parallel conversion, and the parallel complex symbols are shaped with a rectangular pulse and modulated on a subcarrier. In multi-carrier modulation, a frequency spacing between subcarriers is set such that subcarrier modulated parallel (simultaneous) complex symbols are orthogonal to each other, and thus do not inherently interfere with each other.
A typical wireless channel may include multiple paths of different effective lengths, each introducing a different delay to a portion of a transmitted signal following that path. Inter symbol interference (ISI) refers to the effect of neighboring symbols on the current symbol and unless it is compensated for properly it can lead to high Bit Error Rates (BER) at the receiver. Therefore, various methods have been developed to increase the communications systems' performance by reducing the effects of the ISI. In a case where an M-ary QAM signal is transmitted through a wireless “fading” channel without using OFDM, if a delay spread occurring due to multipath delay in the channel is greater than the symbol period of the QAM signal, inter-symbol interference (ISI) occurs and hinders a receiver from restoring the signal normally. For this reason, it is necessary to use an equalizer to compensate for a random multipath delay spread. However, it is very complicated to implement this equalizer in the receiver, and also, transmission performance may be degraded due to input noise.
On the other hand, when OFDM is used, since the period of each parallel (simultaneous) complex symbol can be set to be much longer than a delay spread of a channel, inter-symbol interference (ISI) can be greatly reduced. In particular, when a guard interval is set to be longer than the delay spread, inter-symbol interference (ISI) can be completely avoided. Also, when a guard interval is set to be longer than the delay spread, it is unnecessary to implement an equalizer that compensates for a random delay spread caused by multipath delay. OFDM has proven to be very effective for data transmission through a wireless fading channel and has thus been adopted as a standard transmission mode for terrestrial digital television (DTV) and digital audio broadcasting systems in Europe. In addition, OFDM is used very often in data transmission systems using wired channels such as digital subscriber loops (DSLs) and powerline communications in order to alleviate degradation of transmission performance due to multipath reflection occurring in a wired line network environment.
A transmitter in a data transmission system using OFDM includes a channel encoding unit, a modulation unit, and a transmitter channel matching unit. The channel encoding unit converts data to be transmitted into coded data. The modulation unit which converts the coded data into a complex symbol sequence using a mapper. The complex symbol sequence may be M-ary QAM, M-ary phase shift keying (PSK), differential PSK (DPSK) or the like, and converts the sequence of complex symbols into a plurality of parallel (simultaneous) complex symbols through serial-to-parallel conversion, shapes the parallel complex symbols with a rectangular pulse, modulates them on a subcarrier, and modulates the sum of subcarrier modulated signals on a carrier. The transmitter channel matching unit includes an amplifier and an antenna to transmit the carrier modulated signal through a wireless or wired channel.
A corresponding receiver includes a receiver channel matching unit, a demodulation unit, and a channel decoding unit. The channel encoding unit uses convolutional encoding, block encoding, turbo encoding, or other encoding methods, or a combination thereof.
To perform rectangular pulse shaping of the plurality of parallel (simultaneously transmitted) complex symbols and subcarrier modulation, an Inverse Fast Fourier Transform (IFFT) signal processor is implemented in the transmitter's modulation unit based on sampling theorem. A corresponding Fast Fourier Transform (FFT) signal processor is used in the receiver.
In the transmitter of a data transmission system using OFDM, coded data is converted into a complex symbol sequence by the mapper. Due to the operation of a frequency interleaver in the transmitter and a frequency deinterleaver in the receiver, adjacent complex symbols are independently affected by fading. Accordingly, coded data restored in the receiver is prevented from serious performance degradation due to burst loss. However, an information loss probability due to lading is still high, and therefore, degradation of transmission performance is higher than in data transmission through an unfading channel.
Meanwhile, in OFDM using a plurality of orthogonal subcarriers, each subcarrier demodulated in the receiver appears as the product of a data symbol and of frequency nonselective fading (i.e., a frequency response with respect to the subcarrier).
In OFDM using coherent modulation, during data detection, channel fading distortion is estimated with respect to each subcarrier and a result of the estimation is used as a coefficient of a single-tap equalizer to remove fading distortion from the demodulated subcarrier. During this data detection, channel estimation is essential to detection performance and thus has been researched and extensively employed.
For the purpose of facilitating channel estimation in OFDM, a pseudo-noise (PN) sequence is inserted as an equalizer training symbol into a transmitted signal frame and the channel impulse response (CIR) (the channel's fading distortion) is estimated abased upon the measured correlation between the transmitted-received PN sequence and a known local copy of the PN stored in the receiver.
In Time-Domain Synchronous (TDS) OFDM systems, a PN sequence rather than cyclical prefixes (CP) is inserted between data blocks as a guard interval because the PN sequence is also utilized as an equalizer training symbol at an OFDM receiver and thus spectrum efficiency is higher than in OFDM systems using only a CP.
Methods of estimating a channel impulse response (CIR) using a PN sequence (e.g., based on a cyclical (continuous) correlation between a baseband sampled complex signal received by a receiver and local stored PN) have been introduced by B. W. Song, L. Gui, Y. F. Guan, and W. J. Zhang [“On Channel Estimation and Equalization in TDS-OFDM based Terrestrial HDTV Broadcasting System”, IEEE Trans. Consumer Electronics, vol. 51, no. 3, pp. 790-797, August 2005] and J. Wang, Z. X. Yang, C. Y. Pan, J. Song, and L. Yang [“Iterative Padding Subtraction of the PN Sequence for the TDS-OFDM over Broadcast Channels”, IEEE Trans. Consumer Electronics, vol. 51, no. 4, pp. 1148-1152, November 2005].
In the method introduced by B. W. Song et al., CIR is estimated by detecting a correlation peak in the time domain. However, this method can be used only when a maximum channel time-delay spread is smaller than the sum of the length of a pre-amble and the length of a post-amble, which are respectively attached before and after a PN sequence in a transmitted signal. (See FIG. 1)
Linear equalization does not exploit the fact that the transmitted PN equalizer training sequence has a “finite alphabet” structure. Decision Feedback Equalization (DFE) exploits the fact that the transmitted PN sequence has a “finite alphabet” structure. To take advantage of this property, the decision feedback equalizers use past decisions to (iteratively) improve the equalizer performance. When a (Decision-Feedback-Equalization) DFE iteration is used to estimate long-delay echoes, computation is complicated and desirable performance cannot be accomplished. When the method introduced by J. Wang et al. is used, conversion between the time domain and the frequency domain occurs very often, which results in significant complexity. In addition, desirable performance cannot be accomplished even after many DFE iterations. In other words, conventional channel estimation methods using a cyclical correlation have a very high computation complexity and do not appropriately cancel interference during CIR estimation, thus causing a significant loss in performance.